Information processing apparatus

ABSTRACT

An information processing apparatus is provided which is capable of carrying out the higher memory access. The information processing apparatus includes a memory unit of a synchronous DRAM device which serves to output data synchronously with the supplied clock signal, and a control unit for controlling the access to the memory unit in accordance with an instruction issued from a CPU. The clock signal which is to be supplied to the memory unit is outputted from the control unit. The clock signal which has been outputted from the control unit is supplied to the memory unit and also is pulled back to the control unit. The control unit fetches therein the data, which has been outputted from the storage unit, at the timing which is determined on the basis of the pulled back clock signal. As a result, the control unit reduces the difference between a delay of the data which is outputted from the control unit and a delay of the clock signal which is used to determine the timing at which the data is fetched therein.

This application is a continuation of application Ser. No. 08/601,546filed Feb. 14, 1996, now U.S. Pat. No. 5,828,871.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to an information processingapparatus such as a personal computer, a work station or the like, andmore particularly to a method of controlling a memory provided in suchan apparatus.

2. Description of the Related Art

In recent years, an information processing apparatus such as a personalcomputer, a work station or the like has kept on being miniaturized aswell as being promoted with respect to the high performance. Thethroughput of a microprocessor which is the main unit of such aninformation processing apparatus has been rapidly enhanced by beingsupported by the progress of the semiconductor process technology. Thus,the product has appeared which exhibits the high performance at a highoperation frequency while keeping the power consumption low. Along withsuch rapid enhancement of the capability of the microprocessor, thememory is required which can cope with the enhanced throughput of themicroprocessor. Then, as for the new memory device which compensates fora difference between the capability of the microprocessor and that ofthe memory device, a synchronous dynamic random access memory device(hereinafter, referred to as "a synchronous DRAM device" for short, whenapplicable) has appeared in the market.

The synchronous DRAM device serves to carry out the operation of readingout the data every one clock period for the request of reading out thedata synchronously with the given driving clock signal, and has thefeature in which it can cope with the high operation frequency of themicroprocessor. At the present time, the synchronous DRAM device whichcan cope with the driving clock frequency of 100 MHz (one clock signalcorresponds to 10 ns) has been manufactured. As for such a memorycontrol method, there is known the technology which is disclosed inJP-B-60-3699 for example.

In the case where the synchronous DRAM device as described above isapplied to a memory system of the information processing apparatus, forthe performance of an input-output buffer of an LSI for controlling amemory, the temperature and the power source voltage, the dispersion incharacteristics of the synchronous DRAM device against the change in theoperation environment needs to be taken into consideration. Thedispersion in the characteristics of the synchronous DRAM device due tothe change in the operation environment is an obstacle to bringing outthe high operation performance of the synchronous DRAM device and as aresult the driving clock frequency of the current practical memorysystem is limited to about 33 MHz (one clock signal corresponds to 30ns). Therefore, it can be said that under the present circumstances, itis impossible to make the best use of the high speed reading performanceof the synchronous DRAM device as much as possible.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide aninformation processing apparatus which is capable of reading out thedata from the synchronous DRAM device on the basis of the driving clocksignal of the high frequency irrespective of the change in the operationenvironment.

According to the present invention, an information processing apparatusis constructed by including a memory unit for storing the data and acontrol unit for controlling the operation of reading/writing the datafrom/to the memory unit. According to the preferred aspect, the memoryunit outputs the data synchronously with an inputted clock signal. Afirst wiring through which the clock signal is delivered from thecontrol unit to the memory unit, a second wiring through which the datais delivered from the memory unit to the memory control unit, and athird wiring which branches from the first wiring in order to pull backthe clock signal to the memory control unit are respectively distributedbetween the memory unit and the memory control unit. The memory controlunit includes a storage unit for storing the data, which has been readout from the memory unit, at the timing which is determined on the basisof the clock signal which has been pulled back through the third wiring.

More preferably, the branch of the third wiring from the first wiring isprovided in the vicinity of the memory unit, and the clock signal justbefore delivered to the storage unit is pulled back to the control unit,and the control unit fetches the data in the storage unit at the timingof that clock signal.

Each of the data which has been outputted from the storage unitsynchronously with the clock signal to be delivered to the control unit,and the clock signal which has been pulled back to the control unitthrough the third wiring contains both a delay due to the wiring and adelay due to an internal circuit of the control unit. While those delaysare changed by the change in the environment, those delays are containedin the data delivered to the control unit as well as in the clock signalthus pulled back, whereby the delay of the data and the delay of theclock signal pulled back are changed with the same tendency by thechange in the environment. Therefore, it is possible to reduce thevariable difference between the delay of the data and the delay of theclock signal. As a result, the data which has been outputtedsynchronously with the clock signal can be always written to the storageunit at the same timing irrespective of the change in the environment,and hence it is possible to increase the clock frequency. As a result,it is possible to construct the information processing apparatus whichis capable of fetching the data from the storage unit at the high speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of an informationprocessing apparatus according to an embodiment of the presentinvention;

FIG. 2 is a block diagram showing a configuration of the connectionbetween a main memory 1013 and a memory bus control unit 1012;

FIG. 3 is a timing chart showing the timing at the time when asynchronous DRAM device 102 reads out the data;

FIG. 4 is a timing chart showing the latch timing of the data which hasbeen read out;

FIG. 5 is a block diagram showing a configuration of anotherconstructional example of the connection between the main memory and thememory bus control unit;

FIG. 6A is a timing chart showing the operation of reading out the datain the case where the delay of the driving clock supplied to thesynchronous DRAM device is minimum;

FIG. 6B is a timing chart showing the operation of reading out the datain the case where the delay of the driving clock supplied to thesynchronous DRAM device is maximum;

FIG. 7 is a block diagram showing a configuration of still anotherconstructional example of the connection between the main memory and thememory bus control unit;

FIG. 8A is a block diagram showing a configuration of the connectionbetween the main memory and the memory bus control unit to which thegeneral memory control method is applied; and

FIG. 8B is a timing chart showing the latch timing of the data which hasbeen read out.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will hereinafter bedescribed with reference to the accompanying drawings.

FIG. 1 is a block diagram showing a configuration of an informationprocessing apparatus according to an embodiment of the presentinvention. As shown in FIG. 1, an information processing apparatus 1001of the present embodiment includes, but is not limited to, a CPU 1011, amemory bus control unit 1012, and a main memory 1013. The memory buscontrol unit 1012 is connected to a bus 1014. Both the CPU 1011 and themain memory 1013 are connected to the bus 1014 through the memory buscontrol unit 1012. In addition thereto, an input-output control unit1015, an auxiliary storage device control unit 1016, a display controlunit 1017, and a display memory 1023 are respectively connected to thebus 1014. A keyboard 1018, a mouse 1014, and communication equipment1024 which are provided in the outside of the information processingapparatus are respectively connected to the input-output control unit1015. A compact disc 1020 and a hard disc 1021 are respectivelyconnected as external storage devices to the auxiliary storage devicecontrol unit 1016. In addition, a display 1022 is connected to thedisplay control unit 1017.

FIG. 2 is a block diagram showing a configuration of the connectionbetween the main memory 1013 and the memory bus control unit 1012. Inthe figure, reference numeral 102 designates a synchronous DRAM deviceas the storage device constituting the main memory 1013. Referencenumeral 101 designates a memory control LSI as the circuit forcontrolling the operation of inputting/outputting the data to/from thesynchronous DRAM device 102. The memory control LSI 101 is provided inthe memory control unit 1012. While the memory control LSI 101 iscomprised of one LSI in the present embodiment, it is to be understoodthat one LSI is not necessarily required and hence the memory controlLSI 101 may be combined with an input-output controller for controllingthe operation of inputting/outputting the data to/from the CPU 1101, andthe like into one LSI.

The memory control LSI 101 includes, but is not limited to, a clockgenerator unit 103, an output buffer 104, input buffers 202 and 207, anda data latch 108. Each of the output buffer 104 and the two inputbuffers 202 and 207 has the function of delaying a signal by apredetermined period of time to output the signal. Reference numeral2011 designates a pin through which the clock signal which has beenproduced by the clock generator unit 103 is outputted, and referencenumeral 2012 designates a pin through which the clock signal is fetchedin from the outside. In addition, reference numeral 2013 designates apin through which the data which has been outputted from the synchronousDRAM device 102 is received. In addition, while not shown in FIG. 2, thememory control LSI 101 includes, in addition thereto, at least a circuitfor producing control signals such as a row address strobe signal (RAS),a column address strobe signal (CAS) and a write enable signal (WE), acircuit for delivering the data which has been received from the CPU1011 to the synchronous DRAM device 102, and pins through which thosesignals are respectively inputted/outputted. On the other hand, thesynchronous DRAM device 102 is the DRAM device which operatessynchronously with the leading edge of the clock signal supplied fromthe outside and hence includes a pin 2014 through which the clock signalis inputted, and a pin 2015 through which the data is outputted to theoutside. Hereinbelow, in particular, the clock signal which is suppliedto the pin 2014 of the synchronous DRAM device 102 is referred to as thedriving clock signal. In addition thereto, while in particular not shownin the figure, the synchronous DRAM device 102 includes pins throughwhich the control signals such as the row address strobe signal (RAS),the column address strobe signal (CAS), the write enable signal (WE) andthe address signal are respectively inputted/outputted. The pin 2014 ofthe synchronous DRAM device 102 is connected to the pin 2011 of thememory control LSI 101 through the wiring 2016 and hence the clocksignal which has been produced by the clock generator unit 103 isinputted as the driving clock signal through the pin 2014. In addition,the data which has been outputted through the pin 2015 is transmittedthrough the wiring 2018 and then is inputted to the memory control LSI101 through the pin 2013. In the present embodiment, the driving clocksignal which has been supplied from the memory control LSI 101 to thesynchronous DRAM device 102 is pulled back to the memory control LSI 101to be used as the latch clock signal. For this reason, the wiring 2017branches from the middle of the wiring 2016 which is distributed betweenthe pin 2011 of the memory control LSI 101 and the clock pin 112 of thesynchronous DRAM device 102. The wiring 2017 branching from the middleof the wiring 2016 is connected to the pin 2012 of the memory controlLSI 101. By the distribution of this wiring 2017, the clock signal whichhas been outputted through the pin 2011 is pulled back to the inside ofthe memory control LSI 101 again. Incidentally, in FIG. 2, referencenumerals 105, 203 and 206 designates wiring delays on the wirings 2016,2017 and 2018, respectively.

FIG. 3 is a timing chart showing the timing at the time when thesynchronous DRAM device 102 reads out the data. The synchronous DRAMdevice 102 judges the values of the row address strobe signal (RAS), thecolumn address strobe signal (CAS) and the write enable signal (WE) atthe leading edge of the driving clock signal (CLK). The assignment ofthe data address to the synchronous DRAM device 102 is carried out onthe basis of the address signal which is divided into the row addressand the column address. The synchronous DRAM device 102 fetches thereinthe address, which is supplied at the time when the RAS signal is at thelow level, as the row address and the address, which is supplied at thetime when the CAS signal is at the low level, as the column address. Inaddition, as shown in FIG. 3, the synchronous DRAM device 102 can outputcontinuously the data synchronously with the driving clock signal (CLK).For example, in the synchronous DRAM device which can cope with thedriving clock signal having the frequency of 100 MHz, the access timeranging from the leading edge of the driving clock signal up to the timepoint when the new data is outputted is 9 ns, and hence the new data canbe read out every 10 ns.

The description will hereinbelow be given with respect to the operationin the case where the CPU 1011 reads out the data from the main memory1013.

The CPU 1011 instructs the memory bus control unit 1012 to read out thedata from the main memory 1013. Then, the memory control LSI 101provided in the memory bus control unit 1012 produces a reference clocksignal in the clock generator unit 103 provided in the memory controlLSI 101. Then, the reference clock signal is outputted from the pin 2011to the outside through the output buffer 104, and is inputted as thedriving clock signal (CLK) of the synchronous DRAM device 102 throughthe clock pin 2014. In addition, the memory control LSI 101 suppliesboth the RAS signal and the CAS signal to the synchronous DRAM device102. Then, the synchronous DRAM device 102 judges the values of both theRAS signal and the CAS signal with the leading edge of the driving clocksignal as the starting point and outputs the new data from the dataoutput pin 2015. This data is inputted from the pin 2013 of the memorycontrol LSI 101 through the wiring 2018 and then is supplied to the datalatch 108 in the memory control LSI 101 through the input buffer 107. Inaddition, the driving clock signal which has been outputted from the pin2011 is pulled back through the wiring 2017 branching from the middle ofthe wiring 2016 and then is inputted to the memory control LSI 101through the pin 2012. The clock signal which has been pulled back to thememory control LSI 101 is supplied to the data latch 108 through theinput buffer 202 (hereinafter, the clock signal which has been pulledback to the memory control LSI 101 is referred to as the latch clocksignal for short when applicable). The data latch 108 latches the data,which has been inputted through the pin 2013, at the timing of theleading edge of the latch clock signal. The data which has been latchedin the data latch 108 is then delivered to the CPU 1011 at anothertiming corresponding to the clock signal of the CPU 1011. Otherwise,that data is delivered to the peripheral circuits, such as theinput-output control unit 1015, the auxiliary storage device controlunit 1016 and the display control unit 1017, through the bus 1014 on thebasis of the instruction issued from the CPU 1011. Then, the data whichhas been delivered to those peripheral circuits is transmitted from thecommunication equipment 1014 to the external equipment, stored in thecompact disc 1021 or displayed on the display device 1022 for example.

As described above, in the present embodiment, the driving clock signalwhich has been supplied to the synchronous DRAM device 102 is pulledback to the memory control LSI 101 and then is used as the clock signalwhich is used to latch the data outputted from the synchronous DRAMdevice 102. The difference between the delay of the data inputted to thedata latch 108 with respect to the driving clock signal (i.e., the delaytime of the data inputted to the data latch 108 with reference to thedriving clock signal supplied to the pin 2014) and the delay of thelatch clock signal with respect to the driving clock signal (i.e., thedelay time of the latch clock supplied to the data latch 108, withreference to the driving clock signal supplied to the pin 2014) is thesum of the difference between the wiring delays 206 and 203 and thedifference between the delay due to the input buffer 202 and the delaydue to the input buffer 207. Since each of those delays is changed withthe same tendency against the change in the operation environment suchas the ambient temperature, the change in the difference between thedelay of the data and the delay of the latch clock signal due to theoperation environment becomes very small. Therefore, as shown in FIG. 4,the timing relationship between the data and the latch clock signal iskept approximately constant when the delay of the driving clock ismaximum as well as when the delay of the driving clock is minimum (inthe figure, reference numeral 113 designates the timing at which thedata is outputted from the synchronous DRAM device 102 and referencenumeral 115 designates the timing at which the data is outputted fromthe data latch 108). In order to latch the data at the same timing atall times, the clock period needs to be larger than the differencebetween the delay of the data and the delay of the latch clock signal.According to the structure of the present embodiment, that differencecan be made smaller than the minimum period with which the synchronousDRAM device 102 can cope. As a result, the driving clock signal can beset to the maximum clock frequency with which the synchronous DRAMdevice can cope, and also it is possible to realize the page cycle ofone address/one clock in which the function of the synchronous DRAMdevice is fully exhibited.

FIG. 5 is a block diagram showing a configuration of the connectionbetween the main memory 1013 and the memory bus control unit 1012according to another embodiment of the present invention. In the presentembodiment, the configuration of the memory control LSI is differentfrom that of the memory control LSI shown in FIG. 2. A memory controlLSI 210 includes, but is not limited to, the clock generator unit 103, asystem sequencer 301, a memory read sequencer 302, a read data path 303,a burst data buffer 304 and a selector 305. Incidentally, in the presentembodiment, the whole configuration of the information processingapparatus is not changed largely from that shown in FIG. 1.

The clock generator unit 103 receives the signal which has been inputtedfrom the CPU through both a pin 2246 and an input buffer 2248 and thenproduces the reference clock signal. Then, the reference clock signalthus produced is outputted through an output buffer 2106 from a pin 2131and then is inputted as the driving clock signal through the wiring 2241to the synchronous DRAM device 102. In addition, the reference clocksignal is also inputted through a clock buffer 2136 to the systemsequencer 301. Then, the system sequencer 301 produces the CAS signal onthe basis of the reference clock signal which has been inputted throughthe clock buffer 2136. Then, the CAS signal which has been produced bythe system sequencer 301 is outputted through an output buffer 2107 froma pin 2132 and then is inputted through the wiring 2242 to thesynchronous DRAM device 102. Then, the synchronous DRAM device 102outputs the data synchronously with the driving clock.

The data which has been outputted from the synchronous DRAM device 102is transmitted through the wiring 2245 and then is inputted through apin 2135 to the memory control LSI 2101. Then, the data which has beeninputted from the pin 2135 is passed through an input buffer 2111 and issubjected to the error inspection and the like in the read data path 303and then is inputted to the burst data buffer 304.

The driving clock signal which has been outputted from the pin 2131 ispulled back to the memory control LSI 2101 through the wiring 2243branching from the middle of the wiring 2241 and then is fetched in thememory control LSI 2101 through the pin 2133. Then, the driving clocksignal which has been fetched in the memory control LSI 2101 through thepin 2133 is supplied to the memory read sequencer 302 through the inputbuffer 2108. Likewise, the CAS signal which has been outputted from thepin 2132 is also pulled back to the memory control LSI 2101 through thewiring 2244 branching from the middle of the wiring 2242 and then issupplied to the memory read sequencer 302 through both the pin 2134 andan input buffer 2109. Then, the memory read sequencer 302 judges the CASsignal, which has been pulled back, on the basis of the clock signalwhich has been pulled back and after detection of the CAS signal,outputs a signal to a signal line 114 every one clock after a lapse oftwo clocks. Buffers the number of which corresponds to the length of thecontinuous access of the synchronous DRAM device 102 (in this example,the four buffers 2102, 2103, 2104 and 2105) are provided in the burstdata buffer 304. The data which has been continuously outputted from thesynchronous DRAM device 102 is stored in turn in the respective buffersat the timing of the signal which has been supplied through the signalline 114 from the memory read sequencer 302. For example, in the casewhere the four data D0, D1, D2 and D3 is continuously outputted from thesynchronous DRAM device 102, the data D0, D1, D2 and D3 is stored in thebuffers 2102, 2103, 2104 and 2105, respectively.

The system sequencer 301 outputs the signal every one clock after alapse of five clock signals of the produced CAS signal at the timing ofthe reference clock signal which has been supplied through the buffer2136. The selector 305 is switched on the basis of that signal such thatthe data stored in the buffers 2102 to 2105 provided in the burst databuffer 304 is outputted in turn. The data which has been selected by theselector 305 is outputted through the output buffer 2249 from the pin2247.

FIGS. 6A and 6B are respectively timing charts each showing the timingof the signals in the present embodiment. FIG. 6A shows the basic timingin the inside of the memory control LSI, the timing of the signals onthe memory side (the memory signals timing), and the timing of thesignals with respect to the read out of the data on the memory controlLSI side (the read buffer signals timing) in the case where the delay ofthe driving clock signal which is supplied to the synchronous DRAMdevice 102 is minimum. As for the basic timing, there are respectivelyshown the timing of the reference clock signal (CLK) which is producedin the clock generator unit 103, the timing of the CAS signal which isproduced in the system sequencer 301, the timing of the switching signalwhich is supplied to the selector 305, and the timing of the data whichis outputted through the selector 305. As for the memory signals timing,there are respectively shown in the order from the upper side the timingof the driving clock signal (MCLK) which is inputted to the synchronousDRAM device 102, the timing of the CAS signal (MCAS) which is inputtedthereto, and the timing of the data which is outputted from thesynchronous DRAM device 102. In addition, as for the read buffer signalstiming, there are respectively shown in the order from the upper sidethe timing of the clock signal (MCLK IN) which is supplied to the memorysequencer 302, the timing of the CAS signal (MCAS IN) which is suppliedto the memory sequencer 302, the timing of the switching signal which isoutputted from the memory sequencer 302, the timing of the input datawhich is inputted to the read data path 303, and the data states in thebuffers 2102, 2103, 2104 and 2105. In addition, FIG. 6B shows the basictiming in the inside of the memory control LSI, the memory signalstiming, and the read buffer signals timing in the case where the delayof the driving clock signal which is supplied to the synchronous DRAMdevice 102 is maximum in the same manner as that in FIG. 6A. Inaddition, in FIGS. 6A and 6B, reference numeral 2302 designates a delayof the driving clock signal with respect to the reference clock signal,and also reference numeral 2301 designates a delay of the clock signal,which is supplied to the memory sequencer, with respect to the drivingclock signal.

In the present embodiment, the two terminals, i.e., the pins 2133 and2134 are provided in the memory control LSI such that both the clocksignal and the CAS signal which are supplied to the synchronous DRAMdevice 102 are pulled back to the memory control LSI 2101 by utilizingthose terminals. Then, on the basis of both the clock signal (MCLK IN)and the CAS signal (MCAS IN) which have been pulled back to the memorycontrol LSI 2101, the fetching timing of the burst data buffer 304 isdetermined. Each of the delay 2301 of the signal (MCLK IN), which havebeen pulled back to the memory control LSI 2101, with respect to thedriving clock signal (MCLK) and the delay of the data, which has beeninputted to the read data path 303, with respect to the driving clocksignal (MCLK) is the delay due to both the wirings distributed betweenthe memory control LSI 2101 and the synchronous DRAM device 102, and theinput buffers. Those two delays are changed with the same tendencyagainst the change in the operation environment such as the ambienttemperature. Therefore, even if the delay 2302 of the driving clocksignal (MCLK) with respect to the reference clock signal (CLK) islargely changed due to the change in the operation environment such asthe ambient temperature, the change in the difference between theabove-mentioned two delays is very small. That is, this means that evenif the period of the driving clock signal is reduced, the data can bestored in the respective buffers at the approximately same timing at alltimes against the change in the operation environment. As a result, thefrequency of the driving clock signal can be made the maximum frequencywith which the synchronous DRAMA device can cope, and hence the highspeed reading operation can be made possible.

FIG. 7 is a block diagram showing a configuration of the connectionbetween the main memory 1013 and the memory bus control unit 1012according to still another embodiment of the present invention.

In the configuration shown in FIG. 7, a point of difference of thepresent embodiment from the above-mentioned embodiment shown in FIG. 5is that both the driving clock signal and the CAS signal which are to besupplied to the memory read sequencer 302 of the memory control LSI 5101are obtained by pulling back both the clock signal and the CAS signal tothe memory read sequencer 302 just before outputted from the memorycontrol LSI 101 not from the outside of the memory control LSI 101 butthrough respective bidirectional buffers 501 and 502. In theconfiguration shown in FIG. 7, the clock signal and the CAS signal whichare supplied to the memory read sequencer 302 contains the delays due tothe input-output buffers 501 and 502 with respect to the clock signaland the CAS signal which are outputted from the memory control LSI 5101,respectively. Therefore, the difference between the delays of the clocksignal and the CAS signal, and the delay of the data is small, and hencein the same manner as that in the embodiment shown in FIG. 5, thefrequency of the driving clock signal can be increased. However, in thepresent embodiment, both the clock signal and the CAS signal which aresupplied to the memory read sequencer 302 do not contain the delays dueto the wirings distributed between the memory control LSI 5101 and thesynchronous DRAM device 102. Therefore, the maximum frequency of thedriving clock signal needs to be set to a lower value than that in theembodiment shown in FIG. 5. On the other hand, in the presentembodiment, the input-output pins which are required for those signalsto be pulled back from the outside of the memory control LSI 5101 (thepins 2133 and 2134 of the memory control LSI 101 shown in FIG. 5) areunnecessary, and hence the advantage is provided in which the number ofpins of the memory control LSI can be reduced.

Next, the memory system to which the general memory control method isapplied will hereinbelow be described by being compared with theabove-mentioned embodiment. FIG. 8A is a block diagram showing aconfiguration of the memory system to which the general memory controlmethod is applied. The memory system shown in FIG. 8 corresponds to theabove-mentioned embodiment shown in FIG. 2.

In the memory system shown in FIG. 8A, the reference clock signal whichhas been produced in the clock generator unit 103 provided in the memorycontrol LSI 101 is outputted through the output buffer 104 from a pin3001 to the outside. Then, the reference clock signal which has beenoutputted from the pin 3001 is supplied as the driving clock signal forthe synchronous DRAM device 102 to a pin 2002. Then, the synchronousDRAM device 102 outputs the new data to a pin 3003 with the leading edgeof the driving clock signal as the starting point. Then, the data whichhas been outputted from the synchronous DRAM device 102 is inputted to apin 3004 of the memory control LSI 101. The data which has been fetchedin the memory control LSI 101 through the pin 3004 is supplied throughthe input buffer 107 to the data latch 108. With the reference clocksignal, which was produced in the clock generator unit 103 and has beenpassed through the clock buffer 109, as the latch clock signal, the datalatch 108 latches the input data at the timing of the leading edge ofthe latch clock signal. Now, both the delay due to the output buffer 104and the delay due to the wiring load 105 on the substrate with respectto the reference clock signal which has been produced in the clockgenerator unit 103 are added to the driving clock signal which issupplied to the synchronous DRAM device 102. In addition, both the delaydue to the wiring load 106 on the substrate and the delay due to theinput buffer 107 with respect to the data which has been outputted fromthe synchronous DRAM device 102 are added to the data which is suppliedto the data latch 108. On the other hand, the delay due to the clockbuffer 109 with respect to the reference clock signal is added to thelatch clock signal.

The value which is obtained by adding the access time of the synchronousDRAM device 102 (i.e., 9 ns for the device having the driving clockfrequency of 100 MHz) to the delay of the driving clock signal with thereference clock signal as the starting point (i.e., the delay time dueto both the output buffer 104 and the wiring load 105) and the delay ofthe data (i.e., the delay time due to both the wiring load 106 and theinput buffer 107) is empirically about 30 ns (the typical value). In thememory system shown in FIG. 8A, the value of the delay of the latchclock signal with the reference clock signal as the starting point needsto be previously adjusted in correspondence to the value of the delaysuch that the data can be latched when the delay is maximum as well aswhen the delay is minimum. In this connection, the adjustment of thevalue of the delay of the latch clock signal is carried out by adjustingthe value of the delay due to the clock buffer 109. However, thesynchronous DRAM device 102 outputs one data only for one clock periodof time of the driving clock signal. Therefore, as shown in FIG. 8B, ifthe difference between the maximum value of the delay of the data andthe minimum value thereof with reference clock signal as the startingpoint exceeds one clock period of time of the driving clock signalsupplied to the synchronous DRAM device 102, the system can not bedesigned so as to fetch therein the data every one clock. For example,even if the synchronous DRAM device 102 having the driving clockfrequency of 100 MHz (the access time is 9 ns) is used in the mainmemory, in the case where the typical value of the delay is about 30 ns,since the difference between its maximum value and its minimum value isalso empirically about 30 ns, the period of driving clock signal can notbe made smaller than 30 ns. As described above, in the case where thememory system is constructed by utilizing the conventionally generalmemory control method, the difference between the maximum value of thedelay of the driving clock signal and the minimum value thereof limitsthe operation frequency of the whole memory system and hence it isimpossible to make the best use of the high speed operation of thesynchronous DRAM device. On the other hand, in the above-mentionedembodiment, the frequency of the driving clock signal can be increasedirrespective of the difference between the maximum value of the delay ofthe driving clock and the minimum value thereof since the difference inthe delay between the clock signal for use in the latch and the data tobe latched is always small.

While the present invention has been described with respect to theconfiguration of the connection between the synchronous memory deviceand the memory control LSI of the memory bus control unit forcontrolling the synchronous memory device, with the synchronous DRAMdevice used in the main memory of the information processing apparatus,as an example in the above-mentioned embodiment, it is to be understoodthat the present invention can be supplied to every memory deviceemploying the synchronous DRAM device. For example, in the informationprocessing apparatus shown in FIG. 1, in the case where the synchronousDRAM device is employed for the display memory 1023, the presentinvention can be applied as the memory control method in the displaycontrol unit 1017. In addition, even in the case where the auxiliarystorage device control unit 1016 or the input-output control unit 1015is provided with the buffer memory employing the synchronous DRAM devicein order to absorb the difference in the data transfer speed between thebus 1014 and the communication equipment 1024, or between the bus 1014and the auxiliary storage device such as the hard disc 1021, the compactdisc 1020 or the like, the memory control method of the presentinvention can also be applied.

As set forth hereinabove with the preferred embodiments as the examples,according to the present invention, the relationship between the dataand the latch clock signal can be kept approximately constantirrespective of the magnitude of the delay of the driving clock signalfor the synchronous DRAM device. As a result, it is possible to realizethe memory access with the page cycle of one address/one clock whichmakes the best use of the high speed operation of the synchronous DRAMdevice.

What is claimed is:
 1. An information processing apparatus, comprising:afirst device which outputs a data signal in synchronism with a clocksignal received via a clock signal line; and a second device including:a clock signal output terminal which outputs said clock signal via saidclock signal line; a clock signal input terminal coupled to said clocksignal line; a data terminal which receives said data signal outputtedfrom said first device in synchronism with said clock signal fed backvia said clock signal input terminal; a clock generator which generatessaid clock signal; and a storage unit which fetches said data signalreceived via said data terminal in synchronism with said clock signalfed back via said clock signal input terminal.
 2. An informationprocessing apparatus according to claim 1, wherein said first deviceincludes a memory unit which stores data to be outputted as said datasignal.
 3. An information processing apparatus according to claim 1,wherein said first device corresponds to a display memory which storesdata to be displayed on a display device, and said second devicecorresponds to a display controller which controls operation of saiddisplay device.
 4. An information processing apparatus according toclaim 1, wherein said first device corresponds to a synchronous dynamicrandom-access-memory (DRAM) device.
 5. An information processingapparatus, comprising:a first device which stores data and which outputsa data signal from the data stored in synchronism with a clock signalreceived via a clock signal line; a second device including: a clocksignal output terminal which outputs said clock signal via said clocksignal line; a clock signal input terminal coupled to said clock signalline; and a data terminal which receives said data signal outputted fromsaid first device in synchronism with said clock signal fed back viasaid clock signal input terminal; and a processor which processes thedata stored in said first device, and which instructs said second deviceto read out the data stored in said first device.
 6. An informationprocessing apparatus according to claim 5, wherein said second devicefurther includes a clock generator which generates said clock signal anda storage unit which fetches said data signal received via said dataterminal in synchronism with said clock signal fed back via said clocksignal input terminal.
 7. An information processing apparatus accordingto claim 5, wherein said first device corresponds to a display memorywhich stores data to be displayed on a display device, and said seconddevice corresponds to a display controller which controls operation ofsaid display.
 8. An information processing apparatus according to claim5, wherein said first device corresponds to a synchronous dynamicrandom-access-memory (DRAM) device.
 9. An information processingapparatus, comprising:a display memory which stores data to be displayedon a display device, and which outputs a data signal from the datastored in synchronism with a clock signal received via a clock signalline; and a display controller which controls operation of said displaydevice in dependence upon said data signal outputted from said displaymemory, said display controller comprising:a clock signal outputterminal which outputs said clock signal via said clock signal line; aclock signal input terminal coupled to said clock signal line; and adata terminal which receives said data signal outputted from saiddisplay memory in synchronism with said clock signal fed back via saidclock signal input terminal.
 10. An information processing apparatus,comprising:a first device which outputs a data signal in synchronismwith a clock signal received via a clock signal line; and a seconddevice including: a clock signal output terminal which outputs saidclock signal via said clock signal line; a clock signal input terminalcoupled to said clock signal line; and a data terminal which receivessaid data signal outputted from said first device in synchronism withsaid clock signal fed back via said clock signal input terminal, whereinsaid first device is a synchronous DRAM device.
 11. An informationprocessing apparatus, comprising:a first device which outputs a datasignal in synchronism with a first clock signal received via a firstclock signal line; and a second device including: a clock signal outputterminal which outputs said first clock signal via said first clocksignal line; a clock signal input terminal coupled to a second clocksignal line branching from said first signal line; a data terminal whichreceives said data signal outputted from said first device insynchronism with a second clock signal received via said clock signalinput terminal; a clock generator which generates said first clocksignal; and a storage unit which fetches said data signal received viasaid data terminal in synchronism with said second clock signal receivedvia said clock signal input terminal.
 12. An information processingapparatus according to claim 11, wherein said first device includes amemory unit which stores data to be outputted as said data signal. 13.An information processing apparatus, comprising:a first device whichoutputs a data signal in synchronism with a first clock signal receivedvia a first clock signal line; a second device including: a clock signaloutput terminal which outputs said first clock signal via said firstclock signal line; a clock signal input terminal coupled to a secondclock signal line branching from said first signal line; and a dataterminal which receives said data signal outputted from said firstdevice in synchronism with a second clock signal received via said clocksignal input terminal; and a processor which processes the data storedin said first device, and which instructs said second device to read outthe data stored in said first device.
 14. An information processingapparatus, comprising: toa display memory which stores data to bedisplayed on a display device, and which outputs a data signal from thedata stored in synchronism with a first clock signal received via afirst clock signal line; and a display controller which controlsoperation of said display device in dependence upon said data signaloutputted from said display memory, said display controller comprising:aclock signal output terminal which outputs said first clock signal viasaid first clock signal line; a clock signal input terminal coupled to asecond clock signal line branching from said first signal line; and adata terminal which receives said data signal outputted from saiddisplay memory in synchronism with a second clock signal received viasaid clock signal input terminal.
 15. An information processingapparatus, comprising:a first device which outputs a data signal insynchronism with a first clock signal received via a first clock signalline; and a second device including: a clock signal output terminalwhich outputs said first clock signal via said first clock signal line;a clock signal input terminal coupled to a second clock signal linebranching from said first signal line; and a data terminal whichreceives said data signal outputted from said first device insynchronism with a second clock signal received via said clock signalinput terminal, wherein said first device is a synchronous DRAM device.16. An information processing apparatus comprising:a synchronous DRAMdevice which outputs a data signal in synchronism with a clock signalreceived via a clock signal line; and a controller including: a clocksignal output terminal which outputs said clock signal to said clocksignal line; a clock signal input terminal coupled to said clock signalline; and a data terminal which receives said data signal outputted fromsaid synchronous DRAM device in synchronism with said clock signal fedback via said clock signal input terminal.
 17. An information processingapparatus according to claim 16, wherein said controller furtherincludes a clock generator which generates said clock signal and astorage which fetches said data signal received via said data terminalin synchronism with said clock signal fed back via said clock signalinput terminal.
 18. An information processing apparatus according toclaim 16, further comprising a processor which processes the data storedin said synchronous DRAM device, wherein said processor instructs saidcontroller to read out the data stored in said synchronous DRAM device.19. An information processing apparatus according to claim 16, whereinsaid synchronous DRAM device is used as a display memory storing data tobe displayed on a display device, and said controller is a displaycontroller which controls said display device.
 20. An informationprocessing apparatus comprising:a memory device which stores data andprovides said data in synchronism with a clock signal; and a memorycontroller which controls operations of said memory device, said memorycontroller comprising a clock signal output terminal which outputs saidclock signal, and a data terminal which receives said data outputtedfrom said memory device in synchronism with said clock signal, whereinsaid memory controller further comprises a clock generator whichgenerates said clock signal, a clock signal input terminal whichreceives said clock signal fed back from said clock signal outputterminal, and a latch which latches data outputted from said memorydevice in synchronism with said clock signal fed back via said clocksignal input terminal.
 21. An information processing apparatuscomprising:a memory device which stores data and provides said data insynchronism with a clock signal; and a memory controller which controlsoperations of said memory device, said memory controller comprising aclock signal output terminal which outputs said clock signal, and a dataterminal which receives said data outputted from said memory device insynchronism with said clock signal, wherein said memory controllerfurther comprises:a clock generator which generates said clock signal; asystem sequencer which generates a first control signal and addresssignals in accordance with said clock signal; a memory read sequencerwhich judges the address signals in accordance with said clock signal togenerate a second control signal; a plurality of data buffers whichstore the data outputted from said memory device in sequence of discretedata units in accordance with said second control signal; and a selectorwhich selects outputs of data stored in said data buffers in accordancewith said first control signal.
 22. An information processing apparatusaccording to claim 21, wherein said memory controller further comprisesa plurality of input/output buffers which temporary store said clocksignal and said address signals for transfer between said memorycontroller and said memory device.
 23. An information processingapparatus according to claim 22, wherein said memory device correspondsto a synchronous dynamic random-access-memory (DRAM) device.
 24. Aninformation processing apparatus according to claim 22, furthercomprising a processor coupled to said memory controller which instructssaid memory controller to write data to said memory device and to readdata stored from said memory device for data processing.